Measurements at temperatures below 1.degree. K. are special, not only because quantum phenomena are present on a macroscopic scale, but also because the small heat capacities of many systems studied preclude measurement techniques which gratuitously introduce energy and modify the state of the subject. In addition, relaxation times are long, and a system, once perturbed, may take minutes or even days to return to a state of equilibrium. Consequently, it is difficult to find an accurate, reproducible, and "well behaved" thermometer that is very sensitive, small, of low heat capacity that is not costly and complex. The magnitude of a temperature, or a temperature change, is significant only in relation to the quantity E/k, or .DELTA.E/k, where k is Boltzmann's constant, and E and .DELTA.E are energies which characterize the phenomenon under investigation.
Below 1.degree. K., the traditional method for measuring temperature has been that of magnetic thermometry. A sample is calibrated at a known temperature in the 1-4.degree. K. region, and then, for lower temperature, a "magnetic temperature" T* is extracted from the inversion of Curie's law. The magnetic thermometer is still used because of its basic simplicity, and also as a verifier and "smoother" of temperature scales derived by other methods. Enhanced detection sensitivity is available through superconducting quantum interference device (SQUID) technology. Other methods include Gamma-ray anisotropy and thermal electric noise thermometry.
Because of their relative low cost, resistance thermometers tend to be accorded highest favor as far as practicality is concerned, but very few types having adequate sensitivity exist. Doped germanium semiconductors or germanium resistance thermometers (GRT's) and carbon resistors are much in use down to temperatures as low as a few millikelvins, the germanium being somewhat more reproducible but in the past much more costly.
Currently, the most commonly used temperature sensor in the range between 0.05.degree. K. and 1.degree. K. is the GRT. GRT's are commercially available from several vendors, such as Lake Shore Cryotronics, Scientific Instruments, Inc., and Cryocal Inc. The standard configuration is a relatively thin crystal of germanium enclosed in a gas filled capsule. These commercially available GRT's cannot be used accurately below approximately 0.05.degree. K.
There are three major problems with the present GRT's in the 0.05.degree. K. to 1.degree. K. temperature range. The first problem is that the presently available GRT's depend on gas in the enclosing capsule to conduct the heat away from the germanium sensing element. Helium-4, which is the commonly provided gas, can be used down to about 0.85.degree. K. while, at a significant cost, helium-3 can be used down to about 0.2.degree. K. Below 0.2.degree. K., no useful gas is available. The loss of thermal conduction through the gas decreases the electrical current that can be used to measure the resistance of the germanium crystal since the crystal will over-heat at a very low current level. As a result, the sensitivity of the temperature measurement decreases at lower temperatures.
The second problem with current GRT's is that the temperature sensitivity depends on the slope of the temperature (T) resistance (R) curve for the sensor. The lower temperature GRT's typically have small R vs. T slopes (1/R.times.dR/dt).
The third problem with present day GRT's is that the electrical leads to the germanium crystals are attached to a small area of the crystal. This small contact area induces large electrical gradients in the germanium crystal that can degrade the performance of the temperature sensor.